This invention relates to beds and, more particularly, to improved mattresses for beds that enhance the quality of sleep.
Normally, everyone spends a large percentage of everyday sleeping and the quality of sleep is important to a person's good health and enjoyment of life. Comfortable mattresses are important in establishing restful sleep. During sleep, a healthy person typically passes through five levels of sleep which include stages I–IV and which additionally includes a REM (Rapid Eye Movement) sleep stage. Stages I and II are the lightest sleep and stages III and IV are the deepest. The REM stage is that level in which sleepers dream and receive the mental health benefits attendant to dreaming. All levels of sleep are important, but stages III and IV are the deepest and most physically restful sleep, when, for example, human growth hormone is secreted. Normal sleep is cyclic passing through the stages from I to IV and back from IV to I and into and out of REM. This sleep cycle is repeated a number of times over a normal sleep period, but can be disrupted due, for example, to body discomfort.
Restfulness and the quality of sleep are dependent upon the comfort of sleepers. When sleepers become uncomfortable, they move to relieve the discomfort and the resulting moves are a normal part of sleep. When sleepers move, they frequently change to lighter levels of sleep (stage I or II) or awaken. The more discomfort sleepers feel, the more they will move and the more time they will spend in lighter and less restful sleep. Good sleeping is normally associated with a low number of body shifts during the sleep period. Bed-induced shifts due to discomfort caused by the bed are a significant cause of poor sleep quality. On conventional mattresses (including feather beds, inner spring mattresses, orthopedic mattresses, waterbeds and the like), most people experience about forty major postural body shifts in the course of a night's sleep. Poor sleepers experience about sixty percent more major shifts than good sleepers. While some shifts during a sleep period are beneficial, the quality of sleep can be greatly improved for many by reducing the number of bed-induced shifts.
There are two major causes of bed-induced shifting that cause poor sleep. The first major cause of shifting is the buildup of pressures on parts of the body and the second major cause of shifting is poor body alignment. Considering the first major cause of shifting, the buildup of pressures results from prolonged lying in the same position. On conventional mattresses, the pressure tends to be greatest on the body's protrusions (such as shoulders and hips) where body tissues are put in high compression against the mattress. High compression tends to restrict capillary blood flow which is recognized by the body, after a period of time, as discomfort. The amount of pressure which causes a discontinuance of capillary blood flow is called the ischemic pressure. The ischemic pressure threshold is normally considered to be approximately thirty mmHg. The discontinuance of capillary blood flow is observable as a red spot on the skin. After pressure is applied, a red spot on the skin is a precursor to tissue damage. When parts of the body (usually shoulders and hips in conventional mattresses) are subjected to pressures above the ischemic threshold, discomfort results and, hence, a person shifts to remove the discomfort and threat to tissue damage.
Considering the second major cause of shifting, poor body alignment results from lateral bending of the vertebral column of the body, particularly for a person in a side-sleeping position. Such lateral bending is typically caused by mattresses that allow sagging of the body. Conventional mattresses allow such sagging regardless of the hardness or the softness of the mattress but the sagging effect tends to be more pronounced on soft mattresses. A sagging mattress allows the waist to drop relative to the rib cage and hips and results in stress to muscles, tendons and ligaments. The stress from a sagging mattress frequently manifests as discomfort or even pain in the lumbar region of the back. Such discomfort causes the sleeper to shift in order to relieve the discomfort.
In U.S. Pat. No. 4,662,012 invented by Torbet, one of the inventors herein, an air mattress is disclosed for supporting a person in a reclining position while maintaining spinal alignment and while maintaining low supporting body surface pressure. The Torbet mattress utilized zones running laterally across the width of the mattress with differing air pressures in the zones longitudinally along the length of the mattress. The Torbet mattress has proved to be ideal for supporting sleepers while minimizing supporting body surface pressure and maintaining spinal alignment.
While the Torbet mattress has established a standard of comfort that has not been achieved by conventional mattresses, the Torbet mattress has not been distributed as widely as possible because of its high cost of manufacture. The superior benefits of the Torbet mattress have generally been available only to those, such as hospitals, sleep clinics and the wealthy, willing to pay a high price.
For the Torbet mattress and mattresses in general, persons of greater body weight tend to sink farther into and depress the mattress more than persons of lower body weight. Body protrusions (such as shoulders and hips) cause the highest depression of the mattress and need to be accommodated. The shoulder of a heavy body resting atop the mattress in a side-lying position should not bottom out, that is, the shoulder should not depress the mattress to the extent that an underlying hard supporting surface is felt.
Mattresses using foam and spring sections have been proposed to reduce the cost of the Torbet mattress. Foam or spring sections alone in mattresses, because of the vertical displacement properties of conventional foams and springs, have not satisfactorily achieved simultaneously spinal alignment and uniform low supporting body surface pressure along the interface between the mattress and the body.
An ideal mattress has a resiliency over the length of a body reclining on the mattress to support the body in spinal alignment, without allowing any part of the body to bottom out, and also has a low surface body pressure over all or most parts of the body in contact with the mattress. Since a reclining body has both varying density and varying contour in the longitudinal direction, the ideal mattress must conform to these variations. With such variations, in order to achieve spinal alignment, the supporting forces in the mattress, under load from the reclining body, must vary along the body to match the varying body density and shape. Also, when the body is in spinal alignment, for an ideal mattress, the supporting pressures in the mattress against the skin must be low. The preferred pressure against the skin of a person in bed for an ideal mattress is generally below the ischemic threshold. The preferred side-lying spinal alignment for a person in bed is generally defined as that alignment in which the spine is straight and on the same center line as the legs and head.
While the general principles of an ideal mattress have been recognized since the Torbet mattress, actual embodiments of mattresses that approach the properties of an ideal mattress at reasonable costs not have been forthcoming. Lateral zones, with varying compression in the longitudinal direction, of springs in spring mattresses are capable of achieving spinal alignment if the mattress is of sufficient depth to allow the shoulders and hips to sink into the mattress to a depth that maintains spinal alignment without bottoming out. However spring mattresses generally do not achieve spinal alignment for the primary reason that the compression forces in springs vary as a function of the vertical depression of the springs in compression. The taller the spring in the relaxed state, the greater is the vertical depression and compression of the spring before the force increases to balance the weight of the part of the body lying on the spring. Thus, a body can sink farther into a tall, weak spring before the weight of the body is balanced than it can sink into a short, firm spring. Although tall, weak compression springs are desirable for reducing body pressure, they tend to have intolerable lateral instability and other problems that result in uncomfortable mattresses.
Conventional single-layer spring mattresses with uniform springs are generally unable to provide the qualities necessary for an ideal mattress. In a two-layer structure, the spring compression rate is decreased if one compression spring in one layer is mounted atop another compression spring in another layer. U.S. Pat. No. 5,231,717 used the two-layer structure in multiple zones extending laterally, with different firmness in zones in the longitudinal direction, to provide bedding systems customized for each person in order to provide spinal alignment for each person's particular size and body density. However, such mattresses with different firmness sections in the top supporting layer (the supporting layer closest to the body) provide an irregular firmness that tends to disturb persons in bed.
While substitutes for the Torbet mattress have been attempted, conventional mattresses having zones made from springs and foam do not have the same properties as the air zones in the Torbet mattress. In a Torbet mattress, the force distribution in a zone as a result of vertical depression (caused by a body part such as a shoulder) tends to be distributed and averaged laterally over the entire zone. Because air is fluid, air pressure in a Torbet mattress tends to be averaged and equally distributed in a zone. By way of distinction, the lateral and longitudinal distribution of forces due to a body part depression (for example, from a shoulder) into foam is more local, more complex and is a function of the displacement properties of the particular foam material used. Simple foam and spring mattresses in single or multiple layers have not provided the comfort and other benefits of the Torbet mattress.
The physical properties of mattress materials include among others Density, Hardness, Tensile Strength, Indentation Load Deflection, Compression Load Deflection, Initial Softness Ratio, Resilience (Elasticity), Compression Modulus, Hysteresis and Durability/Lifetime. These physical properties are described as follows.
Hardness is the resistance against pressure.
Density is the mass per unit volume. Hardness and density are interrelated. When density increases, hardness tends to increase. Generally for lower density materials, a growing loss in hardness arises after repeated loading.
Tensile Strength is the measure of the resistance against stretching and changes in tensile strength are measured as Tensile % and changes in length after applying a tensile force are measured as Elongation %.
Indentation Load Deflection (ILD) is a hardness measurement defined in the ISO 2439 standard. ILD in the standard is defined as the force that is required to compress material a percentage of its original thickness, that is, compressed 25%, 40% and 60% from its original thickness (using in the standard a circular plate of 322 cm2). These ILD's are designated ILD25%, ILD40% and ILD60% 
Compression Load Deflection (CLD) is a hardness measurement defined in the ISO 3386 standard. CLD is defined as the counterpressure (force per surface) in Pascal when the core material is pressed in 25% with a stamp where 1 kPA (kilopascal) equals 10 g/cm2 (grams per square centimeter), Compression Set 75%.
Initial Softness Ratio (ISR) is a hardness measurement defined as the ratio of ILD65%/ILD5%. This measurement somewhat correlates to the initial perception of a person about the comfort of a mattress.
Resilience (Elasticity) is an elasticity measurement defined in the ASTM 3574 standard. Resilience/Elasticity is measured by the “ball-rebound” test where a steel ball is dropped from a height onto the mattress core and the rebound of the ball is measured as a % of a predetermined height.
Compression Modulus (Sag Factor) is a compression measurement defined in the ISO 2439 standard. This sag factor is defined as the ratio of ILD65% to ILD25%. The sag factor somewhat correlates with the perception of a person as to whether the mattress supports the body with more uniform alignment.
Hysteresis is a measurement of the load deformation curve of the load surface. The hysteresis curve is determined by loading and de-loading of a mattress core. A circular plate of 355 mm diameter is used to gradually build a force up to a maximum of 1000 Newtons. The hysteresis represents the amount of energy that is absorbed by the material during loading/de-loading. The higher the absorption of energy by a mattress core, the more strength/energy is required by a person to change position on the mattress. Mattress cores which are too soft, have a low hysteresis which results in higher energy requirements for a person changing position on the mattress core. A low hysteresis value generally results in poor sleeping quality.
Durability/Lifetime is a measurement defined in one method by the EN 1957 standard. In this method, a weight of 1400 Newton is rolled 30,000 times up and down on the mattress core. Afterwards the height (Elevation), hardness, ILD and elasticity of the core are measured. This process is repeated once again and the results are compared with the original values and recorded as a as a % retention. The average incline of the hardness is determined at 210 N, 275 N and 340 N in the load deformation curve. Another measurement is defined by the ISO 3385 (DIN 5374) standard. In this method, a foam sample of 40×40 cm forced with a weight of 750 N for 80,000 times at 70 strokes per minute. Afterwards, the loss of height and the hardness are compared with the original values again as a % retention. Tear is another durability parameter measured in pounds per linear inch (pli) and indicates the energy required to pull a sample apart.
In addition to the technical parameters of ideal mattresses described above, many purchasers and merchants have come to expect beds to have other “standard properties”. For example, an expectation is that mattresses will have standard sizes such as King, Queen, Double and so forth with dimensions that match existing fitted-sheet sizes, frame sizes and other bedding equipment sizes. Further, an expectation is that a mattress will be compatible with a two-part bed formed of a foundation and a mattress which together are suitable for use with standard frames, such as “Hollywood” or “Harvard” frames. Purchasers and merchants expect that a bed when made-up with sheets and blankets will appear flat and uniform. The public expects that a bed will have the support and rigidity suitable for a person to sit on the edge for tying shoes or to sit on the edge for other purposes. While these “standard properties” generally do not add to the suitability of the bed for sleeping, they are nonetheless important for widespread commercial acceptance of mattresses.
A number of additional “attributes” are also important for commercial acceptance of mattresses. A mattress design desirably meets the needs of a large percentage of the population. The greatest demand is for beds that sleep two people side by side where typically, one of the two is larger than the other. Mattress sizes desirably accommodate a large percentage of pairs of people (for example, a large man and a smaller woman) in the population. A large percentage of the population is between the measurements for a 97.5 percentile male Caucasian and a 2.5 percentile female Caucasian. While other ethnic body types may be larger or otherwise different in measurement, most of the size differences for different body types are manifested in the length of legs so that, for purposes of mattress sizing, the ethnic size differences of people tend not to be significant. Mattresses are desirably available as a single integrated package easily installed as part of a bed without need for many separate or custom parts that require tailoring or otherwise increase the complexity of bed distribution and assembly. The number of stock keeping units (SKU's) required for a mattress product line is desirably low so that distribution and sale is efficient. Typically, mattresses are marketable in a family of three consumer prices ranges, namely high, medium and low and it is commercially desirable to have a mattress line that is marketable in those different price ranges.
Developments in the parameters of and manufacturing capabilities for foam and other materials have provided new components for mattresses that can be used to better approach the technical parameters required for an ideal mattress at economical costs and which can be manufactured with expected “standard properties” and with the “attributes” for mattresses that are desired by the public.
In consideration of the above background, there is a need for improved mattresses that better approach the properties of ideal mattresses and that can be economically manufactured while satisfying the public expectations and demands for mattresses.